Droplet actuators are used to conduct a wide variety of droplet operations. A droplet actuator typically includes one or more substrates configured to form a surface or gap for conducting droplet operations. The one or more substrates include electrodes for conducting droplet operations. Liquids that are subjected to droplet operations are typically surrounded by an immiscible filler fluid. When the droplet actuator is configured to form a gap, the gap between the substrates is typically filled or coated with the filler fluid. Droplet operations are controlled by electrodes associated with the one or more substrates. Droplets containing particles, such as beads or cells, may be subjected to various droplet operations on a droplet actuator. Droplets associated with particles may require various methods that may include structures, to be manipulated by the droplet actuator.
Beads, whether or not magnetically responsive have a tendency to settle and form aggregates due to one or more forces that may include gravity, friction, electric and magnetic forces. Aggregation may also occur due to surface interactions between beads or between substances bound to beads or interactions between beads and droplet actuator substrates. Regardless of the causes, aggregation has a direct impact on the performance of assays. Immunoassays for example, has critical time consuming stages like incubation and washing that may be influenced by the aggregation of beads.
During incubation, where interaction of different antibodies and antigens result in binding events, the available surface area on the beads for binding is reduced due to aggregation, thereby impeding reaction kinetics and consequently increasing time to result and/or reducing assay sensitivity. Protocols used for incubation, including but not limited to duration of incubation may be influenced by the mixing efficiency within the droplets and also the reaction and binding kinetics, all of which may be impacted by bead aggregation. When it comes to washing, unwanted unbound substances that are trapped in the interstices of bead aggregates are difficult to separate, remove or wash away, thereby resulting in reduced assay sensitivity. Time to results is impacted if more number of washes are required.
Therefore, there is a need in droplet actuators for resuspending and/or circulating beads within a droplet to break up or loosen up aggregates when required to improve the overall assay performance without having to compromise on sensitivity and the overall time to result.